California Drinking Water

How the Combination of Multiple Contaminants Raises Cancer Risks

The array of toxic pollutants in California drinking water could in combination cause more than 15,000 excess cases of cancer, according to a peer-reviewed study by scientists at Environmental Working Group – the first such study to assess the cumulative risk from carcinogenic drinking water contaminants.

For an article published today in the journal Environmental Health, EWG scientists analyzed state and federal data on carcinogens and other toxic contaminants that were detected from 2011 to 2015 in more than 2,700 California community water systems.

They developed a groundbreaking method of calculating the combined health impacts of multiple contaminants in a single water supply.

EWG found that the greatest risks tended to be in small to midsize communities, highlighting that these places are often the most in need of costly treatment systems and other infrastructure to ensure safe drinking water. The greatest risks were from arsenic, byproducts of disinfectant chemicals, and hexavalent chromium, or chromium-6, the notorious “Erin Brockovich” chemical.

These Contaminants in California Tap Water Pose More Than 1-In-1-Million Cancer Risk

Contaminant or contaminant group*

Estimated lifetime cancer cases in California




Hexavalent chromium


Group of 9 disinfection byproducts


Radioactive elements group (uranium and radium)


Group of 5 carcinogenic volatile organic compounds




* Estimated number of lifetime cancer cases for a contaminant group is calculated by summing the estimated number of cancer cases due to individual contaminants in a group.

Source: EWG, from California OEHHA and U.S. EPA IRIS data for 2011 to 2015. The contaminants listed are those whose levels exceeded the one-in-a-million cancer risk in at least 20 water systems. Detailed data on each chemical are in the Methodology.

Drinking water rarely contains only one contaminant, yet regulators currently assess the health hazards of tap water pollutants one by one. This ignores the combined effects of multiple pollutants, which is how people ingest them in the real world. Regulators commonly use the cumulative risk approach to assess the health impacts of multiple air pollutants, but the EWG study is the first known use of this method for drinking water contaminants.

The U.S. Environmental Protection Agency publishes a cumulative risk assessment of carcinogenic air pollutants, known as the National Air Toxics Assessment. The EPA has also proposed evaluating some contaminants by groups, such as volatile organic compounds to make the regulatory process more efficient. EWG’s study builds on these concepts to assess tap water contaminants.

For California water systems, we compared contaminant levels to the cancer risk benchmarks published by the state Office of Environmental Health Hazard Assessment, or OEHHA, and the EPA’s Integrated Risk Information System. The benchmarks are the levels that scientists calculate pose a one-in-a-million risk of cancer – the chance that one person out of a population of one million will develop cancer if he or she drinks the water for a 70-year lifetime.

Legal doesn’t always mean safe

Most U.S. drinking water systems meet all state and federal legal limits. In California, 90 percent of systems met all federal standards for the past seven years, according to the state’s Safe Drinking Water Information System. But legal doesn’t always mean safe.

Legal limits are based on economic and political considerations that usually don’t reflect the lower levels that scientists have found pose health risks. Indeed, over 85 percent of the cancer risk calculated in the EWG study is due to contaminants that were below legal limits. Legal limits may also be based on outdated science: No new contaminants have been added to the list of nationally regulated drinking water pollutants in two decades.

The study found:

  • About 3.1 million Californians get their tap water from 495 systems in which contaminants pose a cumulative lifetime cancer risk greater than one additional case per 1,000 people. In those communities, typically small to medium size, an estimated 4,860 people could develop cancer from drinking their tap water.
  • The largest group of Californians – about 28.5 million – get their tap water from 1,177 systems in which contaminants pose a cancer risk of one per 1,000 to one per 10,000 people. In those communities, an estimated 10,427 cases of cancer could be due to contaminants tap water.
  • Statewide, nearly two-thirds of drinking water systems contained at least two cancer-causing contaminants in excess of one-in-a-million risk levels.

Cumulative Cancer Risk From Tap Water Contaminants in California Water Systems

Cumulative cancer risk

Number of community water systems

Exposed population

Estimated lifetime cancer cases

More than 1 per 1,000




1 per 1,000 to 1 per 10,000




10 to 100 per 1 million




Less than 10 per 1 million








Source: EWG, from “Applying a cumulative risk framework to drinking water assessment: a commentary,” Environmental Health, April 30, 2019.

It’s clear that existing national and state drinking water standards fail to address the health impacts of exposure to multiple pollutants that may be present simultaneously in drinking water. It’s also clear that the federal government’s approach of regulating one contaminant at a time is slow and inefficient.

Federal laws governing the quality of both drinking water and water resources overall must be strengthened to limit tap water contamination and modernize our aging water infrastructure. In the absence of federal leadership, states should take steps to set and enforce drinking water standards that are more rigorous and health-protective than those required by the EPA. Ensuring a safe water supply is a fundamental responsibility of government, and we must demand that public officials at every level step up and fix the badly broken system.

What should the state of California and community water systems do?

Future applications of the cumulative assessment framework developed by EWG would advance water treatment strategies for multiple contaminants in a drinking water supply and help protect public health. The economic benefits of removing multiple contaminants with one treatment technology will help to inform decision-making.

Since 2012, California has championed the Human Right to Water as an essential framework to help communities that lack access to safe and affordable drinking water. Using the cumulative risk approach to assess communities’ water quality will help direct resources where they are most needed. 

For small and midsize communities, finding affordable resources for installation of new and improved water treatment technologies is often difficult. The new risk assessment methods used in the EWG study can help communities and water utilities better evaluate the benefits of water treatment technologies that reduce multiple contaminants simultaneously.

What can you do now?

Individuals and families can take steps to make sure they and their families are drinking the cleanest, safest water possible. Use the EWG Tap Water Database to see whether contaminants were detected in your water. If harmful contaminants were found in your water, even at levels below the federal legal limits, EWG highly recommends filtering your water.

For a detailed explanation of our data and calculations, see the Methods and Data section below.

Methods and Data

For the cumulative cancer risk assessment, we included all contaminants whose average concentration exceeded the one-in-a-million risk level in more than 20 community water systems in California. The cancer risk benchmarks were obtained from the websites of the California Office of Environmental Health Hazard Assessment, or OEHHA, and the U.S. Environmental Protection Agency Integrated Risk Information System.

Contaminants in California Tap Water That Pose More Than 1-In-1-Million Cancer Risk


Drinking water concentration corresponding to 1-in-1-million lifetime cancer risk*

Public water systems with contaminant levels exceeding the one-in-one-million

cancer risk

Population exposed to contaminant

at levels exceeding the 1-in-1-million cancer risk level


Estimated number of lifetime cancer cases for California


Agency defining 1-in-1-million lifetime cancer risk level


0.004 ppb




OEHHA, 2004

Hexavalent chromium

0.02 ppb




OEHHA, 2011


0.06 ppb




OEHHA, 2018 (proposed)


0.1 ppb




OEHHA, 2018 (proposed)


0.4 ppb




OEHHA, 2018 (proposed)


0.5 ppb




OEHHA, 2018 (proposed)

Trichloroacetic acid

0.5 ppb




U.S. EPA IRIS, 2011

Dichloroacetic acid

0.7 ppb




U.S. EPA IRIS, 2003


0.1 ppb




OEHHA, 2009

Sum of uranium-234, uranium-235 and uranium-238

0.43 pCi/L




OEHHA, 2001


0.019 pCi/L




OEHHA, 2006


0.05 pCi/L




OEHHA, 2006


0.0017 ppb





OEHHA, 1999


0.0007 ppb




OEHHA, 2009


0.06 ppb




OEHHA, 2001


0.35 ppb




U.S. EPA IRIS, 2013


0.5 ppb




U.S. EPA IRIS, 2011

* Drinking water concentration corresponding to 1-in-1-million lifetime cancer risk obtained from the websites of the California OEHHA and the U.S. EPA Integrated Risk Information System. Concentrations expressed in parts per billion (ppb) or in picocuries per liter of water (pCi/L), which is a measure of radioactive activity.

Source: EWG, from California OEHHA and U.S. EPA IRIS data for 2011 to 2015. Contaminants listed are those with levels that exceeded the one-in-a-million cancer risk in at least 20 communities.


Chemical concentration corresponding to one-in-a-million risk level (also presented as 1x10-6 risk) is viewed by state and federal health agencies as posing de minimis risk.

The expected lifetime cancer risks for individual contaminants can be calculated by dividing the exposure levels by the one-in-a-million cancer risk level:

Lifetime cancer risk for a contaminant = [Exposure] / [Contaminant concentration corresponding to 10-6 lifetime cancer risk]

And the expected number of lifetime cancer cases can be incorporated by including the populations served by the water system:

Estimated number of lifetime cancer cases for a water system, a contaminant or for the entire state = S  [lifetime cancer risk for a contaminant or group of contaminants] x [population served by the water system or systems]

In this formula, exposure is represented by the average contaminant concentration, and overall cancer risks are treated as additive.

For the cumulative assessment, we calculated the representative contaminant concentrations for each individual water utility represented by the arithmetic means for all available test results for a contaminant within the time frame analyzed.

Data sources

For federally regulated water contaminants that are monitored annually, we used the 2015 data. For contaminants monitored once in several years, we used a longer time frame: 2010 to 2015 for radiological contaminants and 2013 to 2015 for arsenic. And for unregulated contaminants monitored in the EPA’s UCMR 3 program we used the entire data range of 2013 to 2015. Test results reported as non-detects were assigned a value of zero and included in the overall data array for the calculation of averages.

Data for populations served by utilities were obtained from California Safe Drinking Water Information System. Adding the total population served that is listed for the systems in our dataset, we found that population data overestimated by 12 percent the number of California residents served by public water systems, according to OEHHA’s information on how many Californians use private wells or public water systems. A 12 percent population correction factor was applied to estimates of exposed population.

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